1. Field of the Invention
The present invention relates to a solid electrolyte fuel-cell device and, more particularly, to a solid electrolyte fuel-cell device comprising a plurality of cathode layers and anode layers formed on a solid electrolyte substrate, wherein a simple structure that does not require hermetic sealing is employed to achieve a compact and thin construction while, at the same time, achieving increased output.
2. Description of the Related Art
Heretofore, fuel cells have been developed and commercially implemented as a low-pollution power generating means to replace traditional power generation such as thermal power generation, or as an electric energy source for electric vehicles that replaces the internal combustion engine which uses gasoline or the like as the fuel. For such fuel cells, much research effort has been expended to increase the efficiency and to reduce the cost.
Fuel cells can be classified into various types according to the method of power generation, one being the type of fuel cell that uses a solid electrolyte. As one example of the fuel cell that uses a solid electrolyte, a fuel cell is known that uses a calcined structure made of yttria(Y2O3)-doped stabilized zirconia as an oxygen ion conducting solid electrolyte layer. This type of fuel cell comprises a cathode layer formed on one surface of the solid electrolyte layer and an anode layer on the opposite surface thereof, and oxygen or an oxygen-containing gas is fed to the cathode layer, while a fuel gas such as methane is fed to the anode layer.
In this fuel cell, the oxygen (O2) fed to the cathode layer is converted into oxygen ions (O2−) at the boundary between the cathode layer and the solid electrolyte layer, and the oxygen ions are conducted through the solid electrolyte layer into the anode layer where the ions react with the fuel gas, for example, a methane gas (CH4), fed to the anode layer, the end products of the reaction being water (H2O) and carbon dioxide (CO2). In this reaction process, as the oxygen ions release electrons, a potential difference occurs between the cathode layer and the anode layer. Here, when the cathode layer and the anode layer are electrically connected by a lead wire, the electrons in the anode layer flow into the cathode layer via the lead wire, and the fuel cell thus generates electricity. The operating temperature of this type of fuel cell is about 1000° C.
However, this type of fuel cell requires the provision of separate chambers, one being an oxygen or oxygen-containing gas supply chamber on the cathode layer side and the other a fuel gas supply chamber on the anode layer side; furthermore, as the fuel cell is exposed to oxidizing and reducing atmospheres at high temperatures, it has been difficult to increase the durability of the fuel cell.
On the other hand, there has been developed a fuel cell of the type that comprises a cathode layer and an anode layer formed on opposite surfaces of a solid electrolyte layer, and that generates an electromotive force between the cathode layer and the anode layer by placing the fuel cell in a mixed fuel gas consisting of a fuel gas, for example, a methane gas, and an oxygen gas. The principle of generating an electromotive force between the cathode layer and the anode layer is the same for this type of fuel cell as for the above-described separate-chamber type fuel cell but as, the whole fuel cell can be exposed to substantially the same atmosphere, the fuel cell can be constructed as a single-chamber type cell to which the mixed fuel gas is supplied, and this serves to increase the durability of the fuel cell.
However, in this single-chamber fuel cell also, because the fuel cell has to be operated at a high temperature of about 1000° C., there is the danger that the mixed fuel gas may explode. Here, if the oxygen concentration is reduced to a level lower than the ignitability limit to avoid such danger, there occurs the problem that carbonization of the fuel, such as methane, progresses and the cell performance degrades. In view of this, there is proposed, for example, in Japanese Unexamined Patent Publication No. 2003-92124, a single-chamber fuel-cell device that can use a mixed fuel gas whose oxygen concentration is adjusted so as to be able to prevent the progress of carbonization of the fuel, while at the same time, preventing an explosion of the mixed fuel gas.
The above proposed fuel-cell device is of the type that is constructed by housing individual fuel cells in a single chamber; on the other hand, Japanese Unexamined Patent Publication No. 6-196176, for example, proposes an apparatus that does not house a fuel cell in such a chamber, and that generates electricity by placing a solid electrolyte fuel cell in or near a flame and thereby holding the solid electrolyte fuel cell at its operating temperature.
The earlier described single-chamber fuel-cell device obviates the necessity of strictly separating the fuel and the air, as was the case with conventional solid electrolyte fuel-cell devices, but has to employ a hermetically sealed construction. Further, to increase the electromotive force, a plurality of plate-like solid electrolyte fuel cells are stacked one on top of another and connected together using an interconnect material having high heat resistance and high electrical conductivity so as to be able to operate at high temperatures. As a result, the single-chamber fuel-cell device constructed from a stack of plate-like solid electrolyte fuel cells has the problem that the construction is not only large but also costly. Furthermore, as the temperature is gradually raised to the high operating temperature in order to prevent cracking of the plate-like solid electrolyte fuel cells, this type of single-chamber fuel-cell device requires a significant startup time, thus causing extra trouble to operate.
By contrast, the electricity generating apparatus described above employs the solid electrolyte fuel cell of the type that directly utilizes a flame; this type of fuel cell has the characteristic of being an open type, the solid electrolyte fuel cell not needing to be housed in a hermetically sealed container. As a result, this type of fuel cell can reduce the startup time, is simple in structure, and is therefore advantageous when it comes to reducing the size, weight, and cost of the fuel cell. Further, as the flame is directly used, this type of fuel cell can be incorporated in a conventional combustion apparatus or an incinerator or the like, and is thus expected to be used as an electricity supply apparatus.
However, in this type of fuel cell, as the anode layer is formed on the outer circumference of a tubular solid electrolyte layer, radical components due to the flame are not supplied, in particular, to the upper half of the anode layer, and effective use cannot be made of the entire surface of the anode layer formed on the outer circumference of the tubular solid electrolyte layer. This has degraded the power generation efficiency. There has also been the problem that, as the solid electrolyte fuel cell is directly heated by the flame, cracking tends to occur due to rapid changes in temperature, and the solid electrolyte fuel cell, if cracked, eventually disintegrates into pieces, resulting in an inability to generate electricity.
In a solid electrolyte fuel-cell device, if the electromotive force is to be increased, a plurality of fuel cells, each constructed by forming a cathode layer and an anode layer on opposite surfaces of a solid electrolyte layer, have had to be fabricated and combined into a stack as shown in FIGS. 5A and 5B. Likewise, in the case of the fuel cell constructed by forming the cathode layer and the anode layer on the inner and outer circumferences of the tubular solid electrolyte layer shown in FIG. 6, as many fuel cells have to be combined as are needed to generate the required electromotive force. As a result, for applications where a high electromotive force is required but a small output current can serve the purpose, the apparatus size is too large, and reductions in size and cost cannot be achieved.
It is accordingly an object of the present invention to provide a solid electrolyte fuel-cell device wherein a plurality of fuel cells are formed on a single plate-like solid electrolyte substrate to construct a fuel-cell device that does not require a hermetically sealed structure, thereby achieving reductions in size and cost, while also achieving increased durability and increased power generation efficiency.